A laser is based upon oscillation of light in a resonator cavity having ends bounded by mirrors. Generally, the wavelength of the light used for a particular laser is a fraction of the length of the resonant cavity so the light can oscillate with many modes concurrently. The modes for oscillation of light in laser resonant cavities are either classified as transverse or longitudinal modes.
Longitudinal modes are associated with the profile of the laser beam in the longitudinal direction and correspond to various resonant frequencies based on how multiple wavelengths of light match the length of the laser cavity. In general for a wide variety of applications, the transverse modes are of more interest than the longitudinal modes. For instance, many applications would benefit from limiting the number of transverse modes, but would not be equally benefited by limiting the number of longitudinal modes since the number of transverse modes affects the intensity pattern of the laser beam and more often influences beam quality.
Transverse modes are associated with cross-sectional profiles of the laser beam that are transverse to the direction of beam propagation. A laser beam typically has more than one transverse mode. If one could take a series of cross-sectional slices out of a transverse mode, each slice being transverse to the direction of beam propagation and the series of slices taken along the length of the transverse mode following the direction of beam propagation, these slices would reveal certain interesting characteristics of transverse modes. Generally, a slice from one transverse mode, for a particular longitudinal position along a laser beam, will differ in its pattern from a slice from another transverse mode at the same longitudinal position. The patterns may be complex, having many separate parts, or may be a single continuous pattern of simple design such as a dot, square, rectangle, etc.
Although a laser will have many theoretical transverse modes of higher order such as TEM.sub.01, TEM.sub.10, TEM.sub.11, TEM.sub.20, TEM.sub.02, TEM.sub.12, TEM.sub.21, TEM.sub.22, etc., a laser will only have one single theoretical lowest order mode, TEM.sub.00, also known as the theoretical fundamental mode. In practice, often the lowest order mode at which a laser is able to operate is not its theoretical fundamental mode.
Ideally, for many applications it is desirable to prescribe the number of operational transverse modes of a laser. Often, it is ideal to prescribe the operational transverse modes of a laser to be solely the theoretical fundamental transverse mode. Other times it is desirable to prescribe the operational transverse modes of a laser to a higher order transverse mode and to exclude the theoretical fundamental transverse mode of the laser from being present as an operational transverse mode in the operational beam of the laser.
For instance, some lasers for cutting certain materials used in packaging tend to work best with a laser beam having a operational transverse mode of order one degree higher than its theoretical fundamental transverse mode. For this packaging related beam, all other transverse modes, including its theoretical fundamental transverse mode, are excluded from being present in the beam. On the other hand, some lasers used to cut metal work best with their theoretical fundamental transverse mode being the only operational transverse mode present in the laser beam.
Still other applications may call for limiting the laser beam to a combination of selected transverse modes that may or may not include the theoretical fundamental transverse mode as one of the operational transverse modes. Many lasers are constructed so that it is rather difficult or impossible to have them operate at their theoretical fundamental transverse mode or other transverse modes while operating with other undesirable transverse modes present. Challenges then, on one hand, exist to exclude many transverse modes from an operational laser beam. On the other hand, other challenges exist to force other transverse modes to be present in the operational laser beam even though a particular construction of a laser may not be readily conducive to have the desired transverse modes as part of the operational laser beam.
Fortunately, for many applications, transverse modes of higher order are generally undesirable, whereas only one or a few transverse modes of lower order are generally desirable. Many times the theoretical fundamental mode of a laser is the only transverse mode that is desirable as an operational transverse mode. Other times, there may be one or a few transverse modes of orders very close to or adjacent to the theoretical fundamental transverse mode that are desirable as operational transverse modes either alone or in combination with the theoretical fundamental transverse mode of the laser.
Conventional methods exist to limit operational transverse modes to certain lower order modes, such as the theoretical fundamental transverse mode of a laser or a small number of lower order transverse modes, while excluding modes of higher order from being present in the laser beam. These conventional methods have had varying degrees of success and include intra-cavity optical telescopes with solid state lasers, extra-cavity optical telescopes with spatial filters, and filters or interferometers built into the electrode structure. Unfortunately, these conventional solutions tend to be complex and can significantly increase manufacturing costs.